The present invention relates to a pattern collation apparatus for performing collation of a pattern such as a fingerprint on the basis of spatial frequency characteristics.
In recent years, fingerprint collation apparatuses are replacing conventional ID numbers or ID cards in fields where personal recognition is required in, e.g., entrance management for computer rooms or important machine rooms, or access management for computer terminals or teller terminals of banks.
FIG. 12 shows an experimental system shown in Toyoda et al., "Fingerprint Identification System using Liquid Crystal Spatial Light Modulators for Phase Modulation", The INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS, PROCEEDINGS OF THE 1993 IEICE FALL CONFERENCE D-287, September 1993 (Reference 1). Referring to FIG. 12, reference numeral 1 denotes a CRT (Cathode Ray Tube) display; 2-1 and 2-2, phase modulation type liquid crystal spatial light modulators; 3, a lens; 4-1 and 4-2, Fourier lenses; 5-1 to 5-3, half mirrors; 6, a total reflecting mirror; and 7, a photodiode. Reference symbol L denotes a laser beam.
In this system, the fingerprint of a finger to be registered (registration fingerprint) is photographed by a CCD camera (not shown) and stored. The fingerprint to be collated (collation fingerprint) is photographed by the CCD camera. As shown in FIG. 13A, the image of the registration fingerprint and the image of the collation fingerprint are simultaneously placed on the left and right sides to form one input image, and this input image is displayed on the screen of the CRT display 1. The input image displayed on the screen of the CRT display 1 causes interference to generate a vertical fringe pattern after transmission through the phase modulation type liquid crystal spatial light modulator 2-1 and the Fourier transform lens 4-1.
With this processing, the spatial frequency is separated by the first-time optical Fourier transform, so that the input image shown in FIG. 13A becomes a Fourier image as shown in FIG. 13B. In this Fourier image, the low-frequency components of the spatial frequency appear at the central portion. When this Fourier image passes through the phase modulation type liquid crystal spatial light modulator 2-2 and the Fourier transform lens 4-2, second-time optical Fourier transform is performed. With this processing, the Fourier image shown in FIG. 13B becomes a Fourier image as shown in FIG. 13C. In this Fourier image, the low-frequency components of the spatial frequency appear at the central portion while the high-frequency components appear on the left and right sides.
If the registration fingerprint matches the collation fingerprint, the light intensities of left and right correlation component areas S1 and S2 in FIG. 13C increase. The photodiode 7 is arranged such that its light-receiving surface is positioned in, e.g., the left correlation component area S1 of the left and right correlation component areas S1 and S2. Therefore, when the light intensity of the correlation component area S1, which is detected by the photodiode 7, is larger than a predetermined threshold value, i.e., when a correlation peak appears, it can be determined that the registration fingerprint matches the collation fingerprint.
However, according to such a conventional fingerprint collation method, the light intensity of the entire correlation component area S1 is detected by the photodiode 7, i.e., the average value of light intensities in the correlation component area S1 is detected. For this reason, the S/N ratio degrades due to the influence of pixels with low light intensities in pixels constituting the correlation component area S1, resulting in a deterioration in collation accuracy. In addition, the collation accuracy also deteriorates due to the illuminance difference in sampling between the registration fingerprint and the collation fingerprint. More specifically, if the illuminance difference in sampling between the registration fingerprint and the collation fingerprint is large, the fingerprint cannot be recognized as that of a person in question.
When the position of the collation fingerprint shifts with respect to the registration fingerprint, the position where the correlation peak appears also shifts. The conventional fingerprint collation method sometimes cannot cope with such a shift because the photodiode 7 is used to detect the correlation peak. More specifically, when the area of the photodiode is decreased to increase the S/N ratio, the positional shift of the correlation peak cannot be solved. When the area of the photodiode is increased to cope with the positional shift, the S/N ratio decreases. Therefore, an increase in S/N ratio is inconsistent with solution to the positional shift.
When a CCD (Charge Coupled Device) element is used in place of the photodiode 7, and the light-receiving area is increased, the above-described degradation in collation accuracy or the positional shift of the collation fingerprint can be coped with this arrangement. However, a CCD is much more expensive than a photodiode, so that the cost of the entire apparatus largely increases.
The conventional fingerprint collation method requires phase modulation type liquid crystal spatial light modulators and Fourier transform lenses. From the viewpoint of accuracy, it is difficult to manufacture an optical system by combining these parts. In addition, since these parts themselves are expensive, the system inevitably becomes expensive.